Foldable Solar Charger, Cloud-Restart Test - i-surplu 21w (IW-FS21W03i-1)

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Foldable Solar Charger, Cloud-Restart Test - i-surplu 21w (IW-FS21W03i-1)

I recently purchased too many of these fold-able type solar chargers.  Some of them have many reviews, and some have very few, but almost no reviews discuss how the panel deals with shady interruptions like clouds.

NOTE: This is only a test of one aspect of this charger, not a full review.

Charge Restart Feature


These solar chargers are marketed toward backpackers, campers, and/or emergency situations.  In my opinion it makes sense to charge a battery bank with one of these, and then charge your mobile phone with the battery bank later on (after dark?).  These chargers can also work directly with your mobile phones, but that is a tricky endeavor. 

Many mobile phones (and some battery banks) will not adapt to the changing power provided by a solar charger.  For example, current generation apple devices will start charging at full power, but if a cloud reduces the output temporarily, the apple device will not resume charging at full power. 

In response to this, some solar chargers have begun adding charge-restart features to their circuitry.  The theory is nice, the panel detects an interruption in solar energy, and briefly cuts power to the usb ports.  This should trigger an apple device to automatically resume charging at full power.


Depending on how the charge-restart feature is implemented, there could be some serious drawbacks.  Ideally the solar charger would reset the usb ports only once, after the cloud has passed.  That means the charger has to notice a sudden increase in sunlight, and reset the device.

Some solar chargers are taking the easy way out, and just cutting the ports off when the output voltage drops below a certain threshold, say 4.4v.  This method means that when a cloud obstructs the sun, power is cycled immediately.  But as soon as your device begins charging, the voltage will drop again, and power will be cycled again.  This will repeat until the obstacle passes.  This method could be very bad for your device.  This method also makes it impossible to trickle-charge your device.

In this test I will try to determine if these solar chargers have a 'charge restart' feature, and if so how well it works.


The i-surplu  (IW-FS21W03i-1, purchased on amazon, Sept 9, 2016)

Some Pictures





The Test

In my test, I tried to obscure the sun as much as possible without cutting power to the device being charged.  For this charger, I closed the third panel over the second panel, leaving only the first panel exposed to sunlight.  Then I used a semi-transparent piece of plastic over the last panel to further reduce the output.  I monitored the charge rate with a simple USB charging meter.

After the charge rate slowed to almost nothing, I removed the plastic and opened the panels back up.  Here are the results:

This charger has voltage based charge restarting.

When the voltage reported by my usb meter reached approx 4.55v, the usb ports would reset.  After the usb ports came back on, if the cloud had not cleared, the powerbank would start drawing power, causing the voltage to drop again, and the port would reset again.  This process would repeat a few times, then the solar panel started leaving the port shut off for longer periods (10s-15s).  During these periods no charging was possible.

NOTE: Every time the port is reset, the phone chimes and the screen turns on.  This will actually discharge the phone faster than if it were just left alone, completely disconnected.

Further testing is needed to see if the charger would continue to slow down the usb port reset cycle.

With an iPhone 6, the port was definitely cycled the entire time the panel was covered by my plastic cloud.  Surely this is not good for the phone.

When the obstacle moves on, and the sun returns to full strength, charging resumes at full power, even for the iPhone.

Bonus Power Meter

This panel has 4 red leds on the front which give you an idea how much power your device is drawing.  They work very well, and I really like this feature a lot!  When all 4 leds are lit, you know your device is charging with at least 1A of current.

NOTE: The meter is visible in direct sunlight, but the red light would not show up my photos for some reason.  (PWM?)


The Specifics

Resting Voltage
3 panels exposed
Charging Tesa PB
3 panels exposed

Charging Tesa PB

1 panel exposed

Charging Tesa PB
1 Panel with Plastic Cloud

5.26v 4.81v/1.57a restart! restart!



In my opinion, the i-surplu 21w works very well on a sunny day.  It does have voltage based charge restarting, but it is not very well implemented, and the usb port reset cycle drains your device faster, and is surely not good for it. When the sky is partly to mostly cloudy, I would only use this charger with power banks.  If you expect ample sunlight, this charger will definitely keep your device charging at full power.

The front facing charge meter is amazing for monitoring your charge

I always recommend you fully test your setup in varying conditions before you rely on it.


EDIT: polishing some sentences

Edited by: netprince on 02/15/2019 - 12:43
will34's picture
Last seen: 41 min 46 sec ago
Joined: 12/18/2012 - 00:12
Posts: 4091

Thanks once again for the review! Wink

This one seems to restart correctly Thumbs Up